1. Field of the Invention
The present invention relates to a thin film transistor (hereinafter referred to as "a TFT") used in a liquid crystal display apparatus and a method of producing the same, and also relates to a liquid crystal display apparatus.
2. Description of the Related Art
In a liquid crystal display apparatus, an active device such as a TFT is provided for each pixel, in order to display an image with high resolution. A number of TFTs can be formed over a large area, so as to be able to control a very large number of pixels.
Conventionally, as a semiconductor layer of a TFT, an amorphous silicon (a--Si) film formed by reactive plasma chemical vapor deposition using radio frequency discharge (RF-PCVD), a polycrystalline silicon (poly--Si) film obtained by first forming an a--Si film by thermal chemical vapor deposition (thermal CVD) and then recrystallizing the a--Si film by solid phase epitaxy or laser annealing, or another type of film has been used.
On the other hand, a silicon film including a microcrystalline structure (.mu.c--Si) is prepared by RF-PCVD under conditions of a high RF power and a high dilution with hydrogen. It is known that, when the .mu.c--Si is used for an amorphous silicon-based solar cell having a pin structure, the release voltage of the amorphous silicon-based solar cell is increased and the photoelectric conversion efficiency is improved. Also it is known that the .mu.c--Si is prepared more easily with a higher RF power and a higher dilution with hydrogen.
In a case where a silicon film including the microcrystalline structure is prepared under conditions of a high RF power and a high dilution with hydrogen, the crystallization of silicon depends on the type of employed substrate and a thickness of the film. For example, Table 1 shows a relationship between a thickness and a dark conductivity of a p-type silicon film which is formed under conditions that the flow rate of SiH.sub.4 including 0.7% B.sub.2 H.sub.5 is 20 sccm, the flow rate of H.sub.2 is 800 sccm, the substrate temperature is 260.degree. C., the RF power density is 0.04 W/cm.sup.2, and the pressure is 100 Pa.
TABLE 1 ______________________________________ Film Thickness Dark Conductivity ______________________________________ 3000 .ANG. 5 .times. 10.sup.-1 (S/cm) 1000 .ANG. 2 .times. 10.sup.-2 (S/cm) 500 .ANG. 5 .times. 10.sup.-11 (S/cm) 200 .ANG. 5 .times. 10.sup.-11 (S/cm) ______________________________________
It is seen from Table 1 that, when the p-type silicon film which is formed under the above conditions has a thickness of 1000 angstroms or more, the dark conductivity thereof is higher than that of the film having a thickness of 500 angstroms or less by about nine orders. As is understood from the above, if the p-type silicon film which is formed under the above conditions has a thickness of 500 angstroms or less, the film is composed of a--Si. If the p-type silicon film has a thickness of 1000 angstroms or more, .mu.c--Si is formed on a--Si having a thickness of 500 angstroms or more.
Table 2 shows a relationship between a thickness and a dark conductivity of an n-type silicon film which is formed under conditions that the flow rate of SiH.sub.4 including 0.5% PH.sub.3 is 20 sccm, the flow rate of H.sub.2 is 1400 sccm, the substrate temperature is 260.degree. C., the RF power density is 0.05 W/cm.sup.2, and the pressure is 110 Pa.
TABLE 2 ______________________________________ Film Thickness Dark Conductivity ______________________________________ 500 .ANG. 1.8 .times. 10.sup.0 (S/cm) 225 .ANG. 1.3 .times. 10.sup.-1 (S/cm) 150 .ANG. 2.2 .times. 10.sup.-6 (S/cm) ______________________________________
It is seen from Table 2 that, if the n-type silicon film which is formed under the above conditions has a thickness of 200 angstroms or less, the film is composed of a--Si. If the film has a thickness more than 200 angstroms, .mu.c--Si is formed on a--Si having a thickness of about 200 angstroms.
As described above, conventionally, in a case where a silicon film is formed by performing depositions successively in a usual P-CVD apparatus, it is difficult to obtain .mu.c--Si by microcrystallizing a--Si within a thickness of 500 angstroms after the start of deposition, even under the conditions for easily forming .mu.c--Si, i.e., under conditions of a high RF power and a high dilution with hydrogen.
Generally in a TFT, the field-effect mobility in a very thin semiconductor layer which is in contact with an insulating film determines the amount of an ON current of the TFT. The thickness of the semiconductor layer is 1000 angstroms or less in a general TFT, and preferably about 200 to 600 angstroms. Therefore, if aft the semiconductor layer of a TFT is formed under conditions of the high RF power and the high dilution with hydrogen, the semiconductor layer is composed of a--Si. Thus, it is considered that the ON current of the TFT cannot be increased.
An article (1) specified below shows a method for forming .mu.c--Si by repeatedly performing a formation of a silicon film and a hydrogen plasma treatment, with a usual RF-PCVD apparatus.
(1) K. Nomoto, Y. Urano, J. L. Guizot, G. Ganguly and A. Matsuda, "Role of Hydrogen Atoms in the Formation Process of Hydrogenated Microcrystalline Silicon", Japanese Journal of Applied Physics Vol. 29, No. 8, August, 1990, pp. L1372-L1375.
According to the method disclosed in the article (1), an a--Si film is first formed only by using SiH.sub.4, and then a hydrogen plasma treatment is performed for the a--Si film. Then, after a predetermined time period, following the completion of the hydrogen plasma treatment, has elapsed, a next a--Si film is formed. However, there is no report that the above method is applied to a TFT.
In order to produce a large-sized liquid crystal display with high resolution, it is necessary to charge a capacitance formed by a liquid crystal layer and a storage capacitor in a short gate switching time period. However, in a TFT which uses the above a--Si film as a semiconductor layer, it is impossible to increase the ON current because of a low field-effect mobility of the semiconductor layer. In such a TFT, it is necessary to increase the size of the TFT so as to increase the ON current. This disadvantageously leads to a decrease in the opening ratio of the liquid crystal display apparatus.
On the other hand, in a TFT using the poly--Si film as a semiconductor layer, it is possible to increase the ON current of the TFT because of a high field-effect mobility of the semiconductor layer. However, in order to obtain a poly--Si film by crystallizing an a--Si film by solid phase epitaxy, it is necessary to perform an annealing process for about ten hours at temperatures of 600.degree. C. or more. Therefore, it is difficult to use a usual glass substrate, and it is impossible to obtain a TFT having a large area. In another case where the poly--Si film is obtained by recrystallizing the a--Si film by laser annealing, it is necessary to use a low-speed and expensive laser annealing apparatus, so that the method is not suitable for mass production.